J . Phys. Chem. 1987, 91, 3137-3140
3137
Photoinduced Charge Separation in a Micelle-Induced Charge-Transfer Complex between Methylviologen and Ethidium Ions. A Picosecond Absorption Spectroscopy Study S. J. Atherton,* S. M. Hubig,* T. J. Callan, J. A. Duncanson, P. T. Snowden, and M. A. J. Rodgers Center for Fast Kinetic Research, ENS Annex 16N. University of Texas at Austin, Austin, Texas 78712 (Received: April 7 , 1987)
Methylviologen and ethidium ions are shown to form a ground-state charge-transfer, EDA, complex in the presence of sodium dodecyl sulfate micelles. A Benesi-Hildebrand plot gives an equilibrium constant of 100 h 20 M-'. Excitation of this complex with 30-ps pulses of 532-nm light leads to charge separation. Using absorption spectroscopy with picosecond resolution we observe ca. 90% charge recombination with a rate constant of (3.4 f 0.9) X 10" s-I.
Introduction
which receives the probe pulse are two pinholes arranged vertically 6 mm apart and positioned such that the probe pulse adresses the first 1 mm of sample in the direction of the excitation pulse. On the side of the holder which receives the excitation pulse a 1 cm X 1 mm horizontal slot has been cut at the same height as the lower pinhole. The excitation pulse enters the sample through this slot. Travelling through the cuvette are two 1-mm-diameter beams; one has passed through the first millimeter of excited sample; the other (reference beam) has passed through unexcited sample. These are led to the entrance slit of an Instruments SA UFS200 spectrograph (200 lines/"). The cathode of an image intensifier is positioned at the focal plane of the spectrograph. The detection array was built in house following the design of Noe6 and consists of a proximity-focussed image intensifier, two fiber optic conduits, and two linear 5 12 photodiode arrays (reticons) about 12 mm apart vertically. Spectral resolution was determined as 0.6 nm per diode. The reticon output is passed via a Tracor Northern 6200 MCA to a DEC PDP 11/70 computer for storage, analysis, and display. Before each experiment the two diode arrays were balanced. The delay line was set such that excitation occurred after the analysis pulse had interrogated the sample (this method allows for the reticon seeing fluorescence produced by the excitation pulse). Twenty five shots were averaged in this position, and a series of correction factors, C, were computed according to
Recently, several reports have appeared in which electron-donor-acceptor complexes have been irradiated with short pulses of light in the EDA complex absorption In polar media this results in rapid charge separation and, in many instances, an almost as rapid recombination process. Arising out of recent work by one of the present authors concerning the charge-transfer quenching of excited singlet states of the ethidium cation (E') by the methylviologen cation (MV2+) under conditions in which both partners were associated with DNA4v5is the fact that E+ and MVZ+form a strong EDA complex in aqueous micellar (sodium dodecyl sulfate, SDS) solutions. No such complex is observed either in DNA solution or in water under comparable conditions of concentration. We report here the results of irradiating this complex with 30-ps pulses of 532-nm light, observed via timeresolved absorption spectroscopy with picosecond resolution. We compare our observations to similar studies of the photoexcitation of other charge-transfer Experimental Section
SDS (BDH specially pure grade), methylviologen dichloride (Sigma), and ethidium bromide (Sigma) were used as received. Water was purified with a Millipore filtration system. All solutions were freshly made for each experiment. Ground-state absorption and emission spectra were recorded with Hewlett-Packard 8450A and Perkin-Elmer LS5 spectrometers, respectively. The picosecond absorption apparatus is shown in Figure 1. The excitation source is the 30-p, 532-nm second harmonic, pulse from a Quantel YG402 mode-locked Nd:YAG laser. Both the second harmonic and residual 1064-nm lines are extracted from the laser. The dichroic mirror, DIC, reflects the 532-nm pulse 90' while the 1064-nm pulse continues undeviated. The 532-nm pulse traverses a variable delay line before being focussed to a horizontal strip approximately 1 cm wide and 2 mm high, at the cuvette holder. Meanwhile, the 1064-nm light is focussed into a 10-cm cuvette containing a 5050 mixture of D 2 0 and H,O to produce a white light continuum. The continuum is focussed through a diffusing glass plate before being recollimated and focussed to a vertical strip approximately 1.4 cm high and 3 mm wide at the sample holder. The probe and excitation pulses are perpendicular to each other at the cuvette. On the side of the cuvette holder
C(n) = I1(n)/Iz(n) I ( n ) is the output voltage of the nth diode; 1 and 2 are used to indicate the two arrays. The absorption spectrum at a given delay line setting was measured by averaging 25 shots and computing the absorption channel by channel according to A(n) = 1% ( M n ) / 4 n ) C ( n ) ) All data presented have been treated in this way. Results and Discussion
Figure 2 shows the absorption spectra of solutions of 1.5 X 1O4 M E+ and 5 X 1O-, M SDS as a function of MVZ+concentration. In the absence of MVZ+the absorption peaks at 514 nm. With increasing MV2+ concentration the absorption at 514 nm decreases, the peak shifts to 540 nm, and we observe an isobestic point at 525 nm. The inset in Figure 1 shows a Benesi-Hildebrand' plot of these data. Although not completely linear this plot yields an equilibrium constant of 100 f 20 M-' for the formation of the E+,MV*+ charge-transfer complex, under the concentration conditions at the micelle surface region.
(1) Masnovi, J. M.; Huffman, J. C.; Kochi, J. K.; Hilinski, E. F.; Rentzepis, P. M. Chem. Phys. Lett. 1984, 106, 20. Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. Soc. 1983, 105, 6167. -.. .
(2) Mataga, N.; Shioyama, H.; Kanda, Y. J . Phys. Chem. 1987, 91, 314. Mataga, N.; Okada, T.; Kanda, Y.; Shioyama, H. Tetrahedron 1986, 42, 6143. (3) Ebbesen, T. W.; Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1984,
( 6 ) Hutchinson, J. A,; Noe, L. J. IEEE J . Quantum Electron 1984, 20, 1353. (7) Benesi, H. A,; Hildebrand, J. H. J . Am. Chem. SOC.1949, 7 1 , 2703. (8) Atherton, S. J.; Baxendale, J. H.; Hoey, B. M. J . Chem. Soc., Faraday Trans. I 1982, 78, 2167.
106, 7400.
(4) Atherton, S. J.; Beaumont, P. C. J . Phys. Chem. 1986, 90, 2252 (5) Atherton, S. J.; Beaumont, P. C. J . Phys. Chem., in press.
0022-3654187 , ,12091-3137$01 .Sol0 I
0 1987 American Chemical Societv -
3138
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987
Letters For the picosecond absorption experiments we used an aerated solution of 5 X M E+, 3 X loF2M MV2+,and 5 X M SDS. These concentrations were chosen to minimize multiple occupancy of E+ in the micelles (according to Poisson statistics), thereby avoiding complications which may arise due to E+,E+ interaction either in the ground or excited states. We also require a high optical density at 532 nm in order to produce a high concentration of photoproducts in the interrogated volume. Finally, we wished to push the equilibrium MVZ+ + E+
Figure 1. Experimental apparatus: D, frequency doubler; DIC, dichroic mirror; IRP, infrared pass filter, SM, stepping motor; FL, focussing lens; H, heat filter: DIF, diffusing glass: DL, diverging lens; CL, cylindrical lens; CH, cell holder; R, reference beam; S, sample beam; SP, spectrograph; RET, reticon arrays.
At these concentrations essentially all E+ is associated with the micelles. The equilibrium constant for binding of MVZ+is high (7 X lo4 M-').* However, at 5 X lo-' M SDS the micelle concentration is 5 X M (taking 8.1 X M for the cmc and an aggregation number of 64). Thus at the higher concentrations of MV2+ used for the Benesi-Hildebrand plot we are beginning to saturate the micelles with MV2+,possibly accounting for the upward curvature of the plot. Coupled with the observation of a clean isobestic point these data lead us to conclude that a 1:1 charge-transfer complex is being formed. Complex formation is induced by the SDS micelles. The surface charge in the Stern layer will decrease the electrostatic repulsion between E+ and MV2+. At the same time coassociation at the micelles increases their local concentrations.
(MV2+ E+) .e
as far to the right as possible in order to minimize contributions from excitation of uncomplexed E+. At these concentrations of SDS, concentrations of MV2+ above 3 X M result in precipitation of fine crystals, presumably the dodecyl sulfate salt of methylviologen. From the equilibrium constant of 100 f 20 we calculate ca. 75% of E+ is complexed a t 3 X lo-* M MV2+. Moreover any free excited E+ formed is rapidly quenched by the high local MV2+concentration at the micelle surface, as shown by fluorescence intensity measurements. Figure 3A shows the difference absorption spectrum observed immediately after excitation, corresponding to a maximum in the absorbance-time profile. There are peaks near 390 and 570 nm, shoulders near 430 and 610 nm, and a bleaching between 500 and 550 nm. We attribute these features to additive absorptions of reduced viologen ions (MV") and oxidized ethidium ions (F2+). The familiar features of MV" are observed (395-nm peak and 610-nm shoulder). The shoulder at 430 nm and the peak at 570 nm arise from E O 2 + as shown by irradiating a solution of 5 X low4 M Cu2+. Cuz+has a high M SDS, and 4 X M E+, 5 X affinity for SDS micelles and quenches E+ excited singlet states by electron t r a n ~ f e r however, ;~ Cu+ does not absorb strongly in this region. The difference spectrum observed, at the same probe pulse delay as for the previous sample, is shown in Figure 3B. The features are two broad bands at ca. 420 and 570 nm, supporting
1.0
0. 90 0. 80 0. 70
0. 60 W
u
z < m
E5 cn
m 4
0. 50 0. 40
0.30 0.20
a. 10
0
0
d
0 Ln d
0 0 Ln
0 Ln
In
0 0
co
0
L n
co
0 0 b
WAVELENGTH (nm) M E+, and 2 X IO-), 4 X lo-), 6 X IO-), 8 X IO-), I O X IO-), M SDS,1.5 X Figure 2. Ground-state absorption spectrum of solutions of 5 X 12 X lo-', 14 X IO-), 16 X IO-), 18 X IO-), and 20 X IO-) M MV2+. Inset, Benesi-Hildebrand plot.
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 3139
Letters
the assignment given for the absorbances in Figure 3A. Similar absorptions due to EgZ+have been observed via nanosecond absorption spectroscopy of the E+,DNA system containing MVZ+ or Cu2+,and by oxidation of E+ by azide radical via pulse rad i o l y ~ i s . ~There , ~ are considerable errors in the absolute values of absorbance below ca. 450 nm caused by the precipitous drop in continuum intensity in this region. We have chosen to measure kinetics in the region 560-700 nm. The time evolution of the absorbance at 570 and 6Q5 nm in Figure 3A are shown in Figure 4.. The onset of the absorbance occurs over ca. 40 ps which is the rise time of our apparatus as shown by the rise of the S1 S , absorption of 1,Cdiphenylbutadiene after 355-nm excitation. Thus electron transfer within the complex occurs immediately dn our time scale. Subsequently, we observe a fast decay (koW = (3.4 f 0.9) X 1Olos-l) to ca. 9% of the initial value, after which the absorbance decays considerably more slowly. Note that the spectra are essentially identical throughout the entire temporal evolution. This is shown by the equivalence of the kinetics a t 570 and 605 nm and by the inset to Figure 4 showing the initial spectrum and at 240 ps after the pulse. Previous work by Kochi et a1.l and Mataga et a1.2 has shown that excitation of the charge-transfer bands of EDA complexes leads to fast (within ca. 100 ps) charge recombination with 100% efficiency unless there is a rapid decomposition pathway for one of the ions. For example, excitation of the EDA complex between anthracene derivatives and tetranitromethane leads to rapid decomposition of the tetranitromethane anion and hence to the observation of ionic species on the nanosecond and microsecond time scales. For the EDA complex between pyrene and pyromellitic anhydride Mataga et al. note 100% efficient back electron transfer if the charge transfer band only is excited; however, the diffusional quenching of excited pyrene by pyromellitic anhydride results in the formation of free ion species. Ebbesen et aL3 irradiated the charge-transfer bands of both the 1:l and 1:2 complexes between MVZ+ and thiocyanate ions, concluding that, whereas irradiation of the 1:l complex results
-
Figure 3. Transient absorption spectra immediately after the pulse: A, 5X M SDS,5 X M E+,and 3 X M MV2+; B, 5 X M SDS,5 X lo4 M E+, and 4 X M Cut+.
0.3 '1
0.D{ 0.3
@I
4
@
0.2
I
4
54 0
1 .
O J
6
600
0
X
0
m
0
I
I
1
I
I
I
100
200
3 00
4 00
5 00
6 00 PS
Figure 4. Decay of absorption of solutions containing 5 X M SDS, 5 X lo4 M E+, and 3 X absorption spectra (A) immediately after the pulse and (B) 240 ps after the pulse.
M MV2+: 0,at 570 nm; X, at 605 nm. Insets,
3140 The Journal of Physical Chemistry, Vol. 91. No. 12, 1987
in 100%back electron transfer with a rate of >6 X lo9 s-I, in the 1:2 complex this process occurs with a rate of ca. 5 X lo8 s-l with only 50% efficiency. Our data suggest two possible conclusions. By analogy with the 1:2 MV2+:SCN- complex, one reaction scheme is (MV2+...E+)
k& e (MV'+ ...E'2+) k2
k3
MV'+
+ E02+
i.e. rapid back electron transfer, k2,is in competition with breakup of the complex, k3. Our observed fast decay rate, kOM,then equals k , k3, and the fraction of absorbance remaining after the fast decay, F,, is given by Fs = k 3 / ( k 2 + k 3 )
+
By extrapolating both the fast and slower decay components to (9) Atherton, S.J.; Beaumont, P. C., unpublished results
Letters time zero, taken to be the center of the absorbance rise, we calculate F, = 0.09 f 0.02. Thus values of (3.0 f 0.8) X lo9 and (3.1 f 0.8) X loios-l are determined for k3 and k,, respectively. Conversely, the more stable absorptions persisting after ca. 100 ps may be due to rapid diffusional quenching of excited free EB (25%of the total) by MV2+on the micelle surface. In this case the rate of back electron transfer in our system is (3.4 f 0.9) X 10" s-'. Work in progress is expected to elucidate which of the mechanisms is operational. Acknowledgment. We thank Jim Lynch for help with software development and Jean-Claude Mialocq for improvements in optical design. The Center for Fast Kinetic Research is supported jointly by the Biotechnology Research Technology Program of the Division of Research Resources of N I H (RR00886) and by The University of Texas at Austin. Partial support was provided by NIH Grant GM31603 (M.A.J.R.).
